1. Field of the Invention
The present invention relates to a method of manufacturing a crystalline thin-film of rare-earth compound by using a physical vapor deposition, such as a vacuum deposition method or a sputtering method and, more particularly, to a method of manufacturing a thin-film of rare-earth pnictide having a crystalline structure of an anti-Th.sub.3 P.sub.4 type (e.g., Gd.sub.4 Bi.sub.3, Gd.sub.4 Sb.sub.3, and Dy.sub.4 Sb.sub.3) in which a composition ratio of rare-earth elements to that of pnicogen elements is 4:3.
2. Description of the Related Art
Rare-earth pnictides having anti-Th.sub.3 P.sub.4 type crystal structures include many pnictides having useful physical properties. Gd.sub.4 Bi.sub.3 and Gd.sub.4 Sb.sub.3 are ferromagnetic materials having Curie temperatures near room temperature and are expected to be used in a variety of devices. Considering practical applications of these compounds, thin-film formation is an indispensable factor.
Rare-earth pnictides having an anti-Th.sub.3 P.sub.4 type crystal structure have been conventionally studied for bulk structures. No report concerning thin-film formation of these materials has been reported.
According to a conventional method of manufacturing a bulk structure of rare-earth pnictides, rare-earth elements and pnicogen elements are sealed in a sealable crucible made of a high melting point metal such as molybdenum in an atomic ratio of 4:3, and the resultant mixture is heat-treated at a high temperature for a long period of time. For example, in order to manufacture Gd.sub.4 Bi.sub.3, as indicated by a phase diagram of a Gd-Bi binary system of FIG. 1, since Gd.sub.4 Bi.sub.3 has a melting point of 1,520.degree. C. and the adjacent Gd.sub.5 Bi.sub.3 has a melting point of 1,275.degree. C., the mixture has been heat-treated at a high temperature of 1,275.degree. C. to 1,520.degree. C. for a long period of time. In order to manufacture Gd.sub.4 Sb.sub.3, as indicated by a phase diagram of Gd-Sb binary system of FIG. 2, since Gd.sub.4 Sb.sub.3 has a melting point of 1,770.degree. C. and the adjacent Gd.sub.5 Sb.sub.3 has a melting point of 1,640.degree. C., the mixture has been heat-treated at a high temperature of 1,640.degree. C. to 1,770.degree. C. for a long period of time.
A crystalline thin film is generally manufactured by depositing a thin film on a substrate by using a physical vapor deposition such as a vacuum evaporation method or a sputtering method and by heat-treating the thin film. In the manufacture of a compound thin film, elements constituting the compound have a difference in melting point, and it is impossible to heat-treat the thin film at a high temperature, as opposed to formation of a bulk structure due to the following reason. The difference in melting point between the elements appears as a difference in vapor pressure during the heat treatment. This difference is increased when the temperature is increased. When a heat treatment at a high temperature is performed as in formation of a bulk structure, low-temperature elements in the thin film are evaporated in a relatively large amount, and it is difficult to maintain a composition to a design value. For example, in Gd.sub.4 Bi.sub.3 and Gd.sub.4 Sb.sub.3, Gd has a melting point of 1,312.degree. C., whereas Bi has a melting point of 271.3.degree. C. and Sb has a melting point of 630.5.degree. C.. When a heat treatment is performed at a high temperature as in formation of a bulk structure, the element composition ratio in the thin film cannot be maintained as a design value.
This is the main reason why a crystalline thin-film of rare-earth pnictides cannot be formed by a conventional technique.
MBE (Molecular Beam Epitaxy) and the like are conventional thin film manufacturing techniques. According to these techniques, the atomic composition of the thin film to be deposited is precisely controlled, and a target compound is directly manufactured simultaneously with deposition of a thin film. This method, however, must employ an expensive manufacturing apparatus. In addition, this method can be used only for thin-film formation of an alloy such as GaAs whose elements are compatible with each other. There is no report in which a crystalline thin film of rare-earth pnictides is manufactured by this method.